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Zenor Diode

jesaire on Wed May 30, 2012 12:28 AM User is offline

Year: 1999
Make: Jeep
Model: Wrang
Engine Size: 4cly
Refrigerant Type: 134
Country of Origin: United States

Senior Members,
I posted earlier regarding a clutch that does not engage, .3 ohms and could possibly be shorted. Can you tell me about the zenor diode they have on wiring schematics that I have come across, meaning where is located? I am blowing fuses .

You may have come across something very similar in your experiences.


Dougflas on Wed May 30, 2012 5:54 AM User is offline

sometimes there is a diode across the clutch winding. It is not a zener type. It is a spike suppressor. It is usually there for spike suppression. The system will operate without it for testing purposes. It protects the computer and other electronics when the clutch cycles on and off.

NickD on Wed May 30, 2012 6:08 AM User is offline

Zener diode is commonly used for arc suppression, but has to be place in series with a silicon diode. Suppression voltage is much greater, but so is the decay time for faster mechanical operation.

But this doesn't answer the question as to what type of suppression is used or where its located at. Or if it is even the problem, rarely go bad. Clutch coils love to go bad, would pull the vehicle connector first and do a current draw test on the clutch first. If that clutch disk was slipping, burns up that coil in a hurry. Have to test it for about 15 minutes to heat it up to make sure the current goes down, not up as it heats. Normally find that diode in the vehicle harness, but just a guess, sometimes easier to trace the clutch wires out than to look in a manual.

emsvitil on Wed May 30, 2012 3:40 PM User is offlineView users profile

It's a flyback diode.

The clutch coil is a huge inductive load.......

flyback wiki


Edited: Wed May 30, 2012 at 3:42 PM by emsvitil

ice-n-tropics on Thu May 31, 2012 2:54 PM User is offline

Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy. How To Air Condition Your Hot Rod

ice-n-tropics on Thu May 31, 2012 3:16 PM User is offline

Jeep typically had a 600 volt rated Motorola diode incorporated into the clutch wiring harness (a few inches from the clutch) to block the negative voltage spike which is generated when the clutch is disengaged. If the clutch polarity is reversed or if the Jeep experiences a voltage spike such as arcing of the battery terminal or arc welding on the vehicle frame, the diode blows. Chrysler changed to a 1000 volt rated diode in recent years.
When the clutch current is stopped to disengage the armature from the clutch pulley rotor, the magnetic field energy generates a negative voltage spike that needs to be blocked from reaching sensitive components such as computers on board. The more quickly the voltage is disconnected, the larger the negative voltage spike, up to 400 volts as measured with a oscilloscope.
Overheating the clutch harness can melt insulation and short the diode. Flexing/bending/stretching the clutch harness can cause diode failure.
Hope this helps,

Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy. How To Air Condition Your Hot Rod

jesaire on Fri June 01, 2012 2:02 AM User is offline

To all, thanks for the detailed responses, I have actually jumped the compressor (fused) and it instantaneously goes to ground... I will have to assume I have a shorted clutch, the reason I say shorted is because I have no resistance to chassis ground. can one tell me if I can just replace the Clutch? It would be a lot cheaper ( I presume).. Again thanks and I will await additional feedback .


NickD on Fri June 01, 2012 5:44 AM User is offline

Really the voltage of the flyback diode isn't that critical, it is its current rating, typically they will toss in a one amp diode based on its surge current rating. A four amp coil once the magnetic field collapses will maintain that same four amp current, and the coil voltage would remain at 12 volts. Current does decrease at an exponential rate and some figure, how many times is the clutch going to be disengaged. But that surge current can break down the P-N junction of the diode short circuiting it.

Diode polarity is reversed when connected across the coil and that is where the voltage rating of the diode is critical. With a 12 V system, maximum reverse voltage with a high speed good conductive diode should never exceed 12V. Using a 3 amp diode that cost a fraction of a cent more would be the proper solution. Also the speed of the diode is equally important, if slow and stupid, that is when you see a high coil voltage when its disengaged. But that is in the forward direction, not the reverse, that can break down the diode.

Not even sure if you can purchase just the clutch coil, could contact Tim, most I have seen is a clutch kit, disk, idler pulley, and coil, costing practically as much as a new compressor with a new clutch kit installed. If the coil form isn't too badly burnt up can reuse that to wind a new coil, same gauge, same number of turns. If you layer wind it with insulation between each wire and soak it in electrical varnish, would last forever. But they don't do that, a random wound coil, non-insulated save the enamel coating on the magnet wire that can vibrate wearing down the enamel for a shorted coil. A slipping clutch disk is usually the main culprit, generates a lot of heat that breaks down the coil. This can be caused by a binding compressor. So figure its best to change all of it.

Use to be able to buy a pair of blower motor brushes for a quarter, but that was like 40 years ago, now want over 200 bucks or more for a throwaway blower motor that is not even repairable.

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